Johnson Creek Basin Hydrologic Monitoring Study

Johnson Creek forms a wildlife and recreational corridor through densely populated areas of the Portland, Oregon, metropolitan area and through rural and agricultural land in unincorporated Multnomah and Clackamas Counties. Johnson Creek has had a history of persistent flooding and water-quality problems. The USGS has conducted streamflow monitoring and other hydrologic studies in the basin since the 1940s.

Surface Water

The surface-water component of the study includes water level and streamflow monitoring at three locations on Johnson Creek and one at the mouth of the tributary Kelley Creek. Streamflow data from these stations are valuable in both a real-time and historic context. Real-time data are used to provide warning of flooding events. Historic data are used to identify and document changes in flow conditions in response to land use changes, as well as to provide the necessary data for flow frequency analyses. 

Water Quality

Water-quality monitoring consists of continuous stream temperature and turbidity, both indicators of watershed health. Stream temperature is monitored at all four streamflow locations. In addition, temperature is also monitored at two locations on Crystal Springs Creek. Stream temperature monitoring helps identify baseline conditions and trends over time. Along with the six temperature sensors, two turbidity sensors are also located in the Johnson Creek Basin. These are located at the Gresham and Milwaukie gages. Turbidity can be used as a surrogate for suspended sediment in the water, which in turn can be related to the presence of certain pesticided in the water. Differences in turbidity between the upper basin (representd by the Gresham gage), and the lower basin (represented by the Milwaukie gage) may be able to be related to land use.

Groundwater

Data collection and analysis in this study continues to build upon the work done in the Portland Basin Groundwater Study allowing a better understanding of the interaction between the aquifer system, springs, and Johnson Creek. Water levels in 31 wells are measured on a quarterly schedule, adequate to provide water- level hydrographs that represent seasonal climatic changes. The water-level data are useful in predicting the timing of inundation of the low-lying Holgate Lake area, increased flow of Crystal Springs Creek due to a rising water table, and to provide understanding of the interaction of the surface water and groundwater systems throughout the basin. Five wells are instrumented with continuous water-level recorders. Continuous recording allows detection of water-level changes that occur in response to specific precipitation (recharge) events and that may result in increased discharge to springs.